This invention pertains to printed circuits, and more particularly to a process for manufacturing a three dimensional printed circuit substrate using a planar mask and image projection photolithography.
To manufacture a conventional printed circuit substrate, a layer of photoresist film is typically bonded over a continuous conductive layer, which is disposed on one surface of a flat substrate. A photographic mask containing the image of a printed circuit pattern is then placed over and in contact with the film of photoresist, and the mask is flooded with an intense light of the proper wavelength. Light passes through the transparent areas of the mask and polymerizes (or depolymerizes, in the case of positive type photoresist) the exposed areas of the photoresist film. The photoresist film is then chemically developed, leaving behind a pattern of photoresist substantially identical to the printed circuit image on the photomask. Through chemical etching, or a combination of electroplating and chemical etching, this printed circuit pattern is then transferred from the photoresist to the conductive layer, resulting in a conductive printed circuit pattern.
The above described process, which is often referred to as contact printing (because the mask is placed in contact with the substrate), works well for flat printed circuit substrates. If the substrate has relief (i.e., is three dimensional) the flat photographic mask used in the contact printing process cannot be placed in contact with all points on the surface of a three dimensional substrate.
Image projection photolithography has been known in the integrated circuit and thin film hybrid art and a block diagram of a typical "projection mask aligner" is illustrated in FIG. 1. Specifically, FIG. 1 is a diagram of Tamarack Scientific Corporation's model 162 Projection Mask Aligner, and this machine can be used, with the appropriate modifications in exposure procedures, to execute the methods described in the "Description of the Preferred Embodiment" below. Referring to this figure, a light source 102 includes a high pressure mercury arc lamp positioned near the focus of an ellipsoidal reflector, which directs the light upward. A dichroic mirror 104 separates the ultraviolet light from the visible and infrared, and reflects the ultraviolet light towards a shutter 106 and a lenticular integrator 108. A turning mirror 110 then reflects the light through a condenser lens 112, whereupon the light strikes photomask 114. Photomask 114 has the image of a circuit pattern on its lower surface, which blocks some of the light coming from condenser 112. The light then passes through a projection lens which focuses the circuit image on the flat surface of an integrated circuit or thin film hybrid 118.
Since mask 114 includes a planar image of the circuit pattern, the projected image on the surface of the integrated circuit or thin film hybrid 118 is only in focus when that surface is positioned precisely at the focal plane of the projection lens 116. If that image where projected onto the surface of a three dimensional substrate, some, if not most, of the points on the surface of the substrate would be out of focus.
Accordingly, it would be desirable if a three dimensional printed circuit substrate surface could be photolithographically defined using a planar image projection system. The methods and apparatus described below accomplish this objective.